Impulse noise in video signals is characterized by abrupt signal aberrations of relatively short duration. The energy spectrum of impulse noise contaminating video signals is generally flat over the bandwidth of the video signal. Conversely the energy spectrum of composite video signal is concentrated near DC and the color subcarrier. That is, the luminance component of composite video signals has relatively high energy at low frequencies and tapers off at about one megaHertz. The energy of the chrominance component is centered about the color subcarrier with a bandwidth of about two megaHertz. There exists a spectral region centered at about 1.8 megaHertz having relatively little video signal energy.
The frequency region about 1.8 megaHertz is optimum for detecting impulse noise since there is a minimum of video signal energy in this region. U.S. Pat. No. 4,398,210 teaches that the detection of impulse noise within this spectral region of composite video signal affords the opportunity of detecting impulses having amplitudes within the dynamic range of the composite video signal. The system of U.S. Pat. No. 4,398,210 includes memory apparatus for concurrently providing vertically aligned video signal from three adjacent horizontal lines. These signals are respectively band pass filtered to pass the spectral band centered about 1.8 megaHertz. Filtered signal from the first and third lines are respectively subtracted from filtered signal from the second line. The respective differences are compared against a reference value, and the filtered signal from the second line is also compared to the reference value. The results of the three comparisons are applied to an AND gate to determine the presence or absence of correlation and thereby the absence or presence of impulse noise.
The system of U.S. Pat. No. 4,398,210 has proven to be an effective impulse noise detector, however it tends to be hardware intensive. That is, it requires two full line memories and relatively complex correlation detection circuitry. The present invention overcomes both of these shortcomings.